1. Field of the Invention
The present invention is directed to a system for blending images.
2. Description of the Related Art
Many industries and applications have been using techniques for combining images. One example of a technique for combining images is chroma keying (also called blue screening). Chroma keying is a process which allows the creation of scenes that would otherwise be too dangerous, impossible or impractical and is one of the most popular special effects used in the motion picture, television and video industries today. Creating a composited image using chroma keying starts with the subject that has been photographed (still or video) in front of an evenly lit, bright, pure blue background. The image of the subject and the blue background is called the foreground. A matte is created which indicates the density and hue of the blue for each pixel in the foreground. The compositing process replaces all of the blue areas in the picture with another image known as the background. The new image created by chroma keying is called the composited image. Chroma key composites can be made for photos and/or videos. The matte is a grey scale image (in some instances a black and white image) that is used to determine the percentage of foreground and background values that will be used for each pixel in the composited image. The matte is also known as an alpha, alpha signal, key or key channel. The chroma keying process is typically performed using a keyer.
Originally, keyers used a non-additive mix. That is, keyers were devices that switched back and forth between the background and foreground to create a composited image. The limitation of this approach is that there is no way a transparent or translucent foreground subject can be added to the background scene in such a way that the background is still visible through the foreground subject. A viewer either sees the foreground object by itself or the background object by itself, depending on the switch. Also, since there is a switch involved, there is a limit to how fast the device can switch back between foreground and background.
The original keyers were then replaced by linear keyers, which combine (or key) the background and foreground in proportions determined by the level of the alpha signal, rather than simply switching between the foreground and background. A linear keyer can make a foreground object appear transparent. Some linear keyers perform an additive mix of the foreground and background. Other linear keyers will attenuate the foreground prior to blending with the background.
One problem with traditional linear keyers is they suffer from blue spill, which is the discoloration of the foreground subject caused by spill light from the blue background. One device which improved on traditional keyers is the Ultimatte from Ultimatte Corporation (www.ultimatte.com). The Ultimatte improves on linear keyers by processing the foreground in order to suppress the backing and remove blue spill, while at the same time permitting many shades of blue to be reproduced in the foreground.
One drawback with keyers, including the Ultimatte, is that they can only key on one color or one contiguous range of colors. That is, these keyers look for blue (or some other single color). Upon finding the color being searched for, the device keys in the background to replace the foreground pixels. Additionally, traditional keyers do not look at the background when determining keys. One disadvantage of not looking, at the background is that if there is no background to replace the foreground, it is not useful to waste resources calculating an alpha.
Another system that has been used to combine images is the famous and successful 1ST & Ten™ virtual yardline system, developed by Sportvision, Inc. (www.sportvision.com). This system adds a virtual first down line to a television broadcast of an American football game. The system first determines where in a video the line should be added. The line is then rendered in a separate frame or field of video. The program video (e.g. showing the football game) is analyzed to determine which pixels can be replaced by the line. The image of the line is then blended with the appropriate pixels of the program video. The determination of how to blend pixels is based on a combination of the program video with prestored color information. That is, an operator has previously determined which colors may be modified in a video. For example, the operator may determine that green relating to the grass, white relating to chalk, and brown relating to dirt can be modified, while colors in the players' uniforms cannot be modified. This system presents an improvement over the prior art because it allows the user to designate many colors to be replaced rather than one color.
The 1ST & Ten™ system analyzes the program video to create an alpha signal using a computer. Using a computer is not optimal. For example, the computer must process the pixels in a frame at the normal NTSC frame rate of thirty frames per second. As such, the system is not able to process every pixel in the foreground frame. Thus, the graphic is added as a series of polygons and only pixels pertaining to certain portions of the polygons are considered when determining the alpha signal. Additionally, because of the timing, the graphic being added to the program feed is limited in number of pixels.
Thus, there is a need for an improved system for combining images.